4 DISCUSSION
This
study shows that plateau pika disturbance is related to lower palatable
plant biomass and higher plant species richness across five sites or at
an individual site, demonstrating that plateau pika disturbance
decreases the forage availability service and increases the biodiversity
conservation service, in agreement with the first hypothesis. Plateau
pika disturbance in relation to lower
palatable
plant biomass has also been reported in a previous
study (Liu et al., 2017), while
prairie dog disturbance has been
reported to be related to higher palatable plant
biomass (Martínez-Estévez et al.,
2013). Prairie dog disturbance encourages the arid grassland in North
America to shift from annual plants to perennial plants, in which blue
gramma (Bouteluoa gracilis ) and vine mesquite (Panicum
obtusum ) are usually perennial plants that are palatable for livestock
(Sierra-Corona et al., 2015), whereas plateau pika disturbance enables
alpine grassland composition to change from sedge to unpalatable
broad-leaved plants in the Qinghai-Tibetan Plateau (Pang & Guo,
2018). In this case, plateau pikas
preferentially consume dicotyledons, and most dicotyledons are
unpalatable plants (Pang & Guo, 2017; Zhao et al., 2013). Thus, plateau
pika consumption can benefit palatable plant growth (Pang & Guo, 2017)
since it can alleviate the competitive pressure of monocotyledons, and
these monocotyledons are palatable plants (Li et al., 2015; Zhao et al.,
2013). However, plateau pikas often
bury many plants in the process of producing bare soil patches, thereby
reducing aboveground plants at the plot scale. The decrease in palatable
plant biomass, which is derived from bare soil patches and sedge
replacement by unpalatable broad-leaved plants, is greater than the
increase in palatable plant biomass derived from the alleviation effect
of plateau pika consumption. Therefore, plateau pika disturbance
contributes to a negative effect on palatable plant biomass, resulting
in a decrease in forage availability services. In addition, plateau pika
disturbance in relation to higher plant species richness is similar to
that of other small burrowing herbivore disturbances, such as that of
the plains pocket gopher (Rogers et al., 2001), the coexisting kangaroo
rat and mole rat (Davidson & Lightfoot, 2008), and the prairie dog
(Hagenah & Bennett, 2013). Higher
plant species richness caused by plateau pikas can be explained by
several mechanisms: first, plateau pika activities are likely to enhance
the environmental
heterogeneity
(Guo et al., 2012b; Pang et al., 2020a; Yu et al., 2017a), which creates
gaps for opportunistic plant species (Hagenah & Bennett, 2013; Pang &
Guo, 2017); second, bare soil patches caused by plateau pikas provide
additional good habitat for plant seeds to germinate and settle (Pang &
Guo, 2017) because it is difficult for plant seedlings to survive on
vegetated surfaces with 7 to 8 cm
root mats with less soil and water supplies (Li et al., 2015); third,
the burrow of plateau pikas is the only habitat home for snow finch
(Montifringilla ruficollis ) (Liu et al., 2013), and this bird can
recruit more plant seeds in areas with plateau pikas. These three
processes encourage plateau pika disturbance to increase the
biodiversity conservation service.
Plateau pika disturbance is
associated with lower soil water storage across all five sites or at
each individual site, in contrast to the second hypothesis. This result
has also been found in a previous study (Liu et al., 2013), whereas it
is different from prairie dog disturbance (Martínez-Estévez et al.,
2013). The different responses of soil water storage to disturbance by
small burrowing herbivores are ascribed to the soil porosity and
compaction of grassland. Prairie
dog disturbance increases soil water storage by reducing the compaction
and increasing the pore spaces of arid grassland (Martínez-Estévez et
al., 2013), while plateau pika disturbance has no impact on the soil
bulk density of alpine grassland (Pang et al., 2020b) and encourages
more topsoil water to infiltrate into deep soil though bare soil patches
and burrow entrances in root mats (Li et al., 2015), contributing to a
decrease in soil water storage in the topsoil
layer. In addition, bare soil
patches caused by plateau pikas can increase water evaporation in
topsoil because of the lack of vegetation insulation. Thus, the plateau
pika disturbance results in low soil water storage in the topsoil layer,
contributing to a reduction in the water conservation service.
Plateau
pika disturbance correlates with higher soil organic carbon stock and
does not relate to soil total potassium stock across the five sites or
individual site, whereas plateau pika disturbance has different impacts
on soil total nitrogen stock and soil total phosphorus stock among five
sites, demonstrating that there is a general pattern for plateau pika
disturbance in relation to carbon sequestration and soil potassium
maintenance services, and no general pattern concerns the effects of
plateau pika disturbance on the soil total nitrogen and total phosphorus
maintenance services. These results support the third hypothesis but do
not support the fourth hypothesis.
In this case, plateau pika
disturbance can increase the soil organic carbon stock in two possible
ways: first, plateau pika disturbance can input extra organic matter
through the deposition of uneaten food (Liu et al., 2009; Liu et al.,
2013; Yu, et al., 2017a; Zhang et al., 2016) and the excretion of urine
and feces (James et al., 2009; Yu et al., 2017b); second, the burrowing
behavior of plateau pikas can protect the soil organic matter pools from
ultraviolet light and cannot be blown away by wind (Pang et al., 2020a),
which reduces the decomposition, mineralization and removal of soil
organic matter (Clark et al., 2016; Yurkewycz et al., 2014). Since the
responses of soil total nitrogen and total phosphorus maintenance to
plateau pika disturbance are site-dependent, which is similar to
previous studies (Pang et al. 2020b), soil total nitrogen and phosphorus
maintenance is not suitable for estimating the effect of disturbance by
small burrowing herbivores on the ecosystem services of alpine
grasslands.
Additionally,
this study found that plateau pika disturbance intensity was also
related to palatable plant biomass, plant species richness, soil water
storage, soil organic carbon stock, and soil total nitrogen and
phosphorus stocks. As the disturbance intensity increases, the palatable
plant biomass, plant species richness, soil organic carbon stock, soil
total nitrogen and phosphorus stocks show downward parabolas,
demonstrating that there is a threshold of plateau pika disturbance
intensity for maximizing the forage availability, biodiversity
conservation, carbon sequestration, and soil nitrogen and phosphorus
maintenance services. When the disturbance intensity is within the
threshold of disturbance intensity, plateau pika disturbance can enhance
soil total nitrogen (Li et al., 2014) and organic carbon accumulation
(Yu et al., 2017b), increase palatable plant biomass (Pang & Guo, 2018)
by improving the growth potential of grass plants (Wang et al., 2012b),
and encourage more hygrophytes and mesophytes, annual and perennial,
common and rare plants to coexist (Guo et al., 2012b), contributing to
higher forage available, biodiversity conservation, carbon
sequestration, soil total nitrogen and phosphorus maintenance services.
When
the disturbance intensity is below the threshold of disturbance
intensity, the dominant sedge plants place great competitive pressure on
grass plants, which leads grass plants to maintain a low percentage
(Pang & Guo, 2018; Wang et al., 2012b) and makes it difficult for rare
plants to coexist. Once the
disturbance intensity surpasses its threshold, low soil water content in
alpine grassland (Liu et al., 2013) only sustains the xerophytes and
mesophytes, most of which are unpalatable (Pang & Guo, 2018), resulting
in reductions in palatable plant biomass and plant species richness.
Notably, the responses of plant species richness to plateau pika
disturbance intensity showed linear increases at Luqu and Gonghe, which
were ascribed to the disturbance intensity. The disturbance intensities
at these two sites were lower than the threshold of disturbance
intensity (Table 2), which validates the general patterns of plateau
pika disturbance intensity in relation to plant species richness.
Plateau pika disturbance can
increase the input of soil organic matter (Pang & Guo, 2017; Pang et
al., 2020a), and this increase in soil organic matter is lower when the
disturbance intensity is below the threshold. Low vegetation biomass at
high disturbance intensities (Pang & Guo, 2017; Pang et al., 2020a; Sun
et al., 2015) also decreases the input of soil organic matter. Thus,
soil organic carbon sequestration and soil nitrogen and phosphorus
maintenance services are low when the disturbance intensity is below or
over the thresholds.
In
contrast to forage availability, biodiversity conservation, carbon
sequestration, soil nitrogen and phosphorus maintenance services, the
water conservation service shows a
decreasing trend as the plateau pika disturbance intensity increases,
which is ascribed to evaporation and water infiltration on bare soil
patches.
Previous
studies have shown that evaporation and water infiltration from topsoil
to deep soil are higher in bare soil patches than in vegetated surfaces
(Liu et al., 2013; Yu et al., 2017b). As the soil bare area increases,
the amount of water evaporation and infiltration shows an increasing
trend (Liu et al., 2013), which encourages the soil water storage to
decrease, contributing to a gradual decrease in the water conservation
services of alpine grasslands.
Data from one site show that prairie dog disturbance has a positive
impact on grassland ecosystem services in arid regions of North America
(Martínez-Estévez et al., 2013), whereas data from five sites show that
plateau pika disturbance and its disturbance intensity have a positive
or negative impact on different ecosystem services of alpine grasslands
in the Qinghai-Tibetan Plateau. Thus, a general pattern can be
identified for the effect of plateau pika disturbance on forage
availability, biodiversity conservation, water conservation and carbon
sequestration services, whereas the soil nutrient maintenance service in
relation to plateau pika disturbance varies by site. These results
indicated that plateau pika control should consider the management
target of ecosystem services for alpine grasslands and its disturbance
intensity threshold in the Qinghai-Tibetan Plateau. The findings of this
study present a possible pattern of how disturbance by small burrowing
herbivores influences grassland ecosystem services and open a broader
vision for insight into small burrowing herbivore in relation to
grassland ecosystem services.
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